As is known, in order to reduce atmospheric pollution, the standards of the majority of countries are imposing increasingly stringent limits upon the composition of the exhaust gases produced by internal-combustion engines.
In particular, as regards diesel engines, the main problems are represented by the presence, in the exhaust gases, of nitrogen oxides (NOx) and of particulate, whereas carbon oxides (CO) and hydrocarbons (HC) do not constitute any particular problem.
Many methods have so far been proposed in order to bring the amount of particulate present in exhaust gases introduced into the environment down to minimum values. Amongst these, however, without any doubt, the provision on the gas-exhaust pipe of a diesel-particulate filter, also known as “soot catcher” or “soot trap”, has been recognized for many years now in the field of automobile engines as the final answer to the problem of emissions of particulate of diesel engines.
In particular, a diesel-particulate filter is in general constituted by parallel channels with alternately obstructed porous walls. The obstructions force the exhaust gases to traverse the side walls of the channels so that the unburnt particles constituting the particulate are first withheld in the porosities of the side walls themselves and then, when the latter are completely filled, accumulate on the internal surfaces of the walls of the channels to form a layer of accumulation.
As the accumulation of particulate increases on the internal surfaces of the walls of the channels, also the drop in pressure on the diesel-particulate filter and hence the counterpressure generated by the diesel-particulate filter itself increases.
The particulate cannot hence be accumulated indefinitely in so far as high accumulations cause:
deterioration of performance, driveability and consumption of the engine, until, in limit conditions, engine itself stalls; and
destruction of the diesel-particulate filter in the case of self-ignition and uncontrolled combustion of the particulate; in fact, in the presence of high accumulations of particulate, in particular driving conditions, phenomena of “critical” regeneration may be triggered, which consist in the sudden and uncontrolled combustion of the particulate: consequently, the high temperatures that are generated within the ceramic matrix of the diesel-particulate filter can cause damage thereto.
It is consequently necessary to remove periodically the particulate accumulated, performing the so-called “regeneration” of the diesel-particulate filter. In particular, in the automobile-engine field, by “regeneration” of the diesel-particulate filter is understood the combustion of the particulate accumulated (prevalently made up of carbon, C), which, in contact with the oxygen present in the exhaust gases, transforms into carbon monoxide, CO, and carbon dioxide, CO2.
However, this reaction occurs naturally (i.e., without the use of additives) only at temperatures higher than approximately 600° C., and said temperature levels are much higher than those that are measured at inlet to the diesel-particulate filter in conditions of normal operation of the engine.
It is consequently necessary, in certain conditions, and namely, when given levels of accumulation of particulate are detected in the diesel-particulate filter, to raise the temperature of the exhaust gases at inlet to the diesel-particulate filter itself artificially until self-ignition of combustion of the particulate is obtained.
Regeneration of a diesel-particulate filter constitutes the main problem linked with use of said type of filters in the automotive field.
In particular, the methods of self-ignition of combustion of the particulate may be broadly divided into two major categories, according to the type of approach adopted: belonging to the first category are methods of self-ignition based upon the use of an additive in the diesel fuel, which, by acting as catalyst, enables a reduction in the temperature for triggering regeneration of approximately 100 to 150° C., whilst belonging to the second category are those methods of self-ignition of combustion of the particulate based upon control of combustion of the engine.
On account of the numerous disadvantages, the methods belonging to the first category have been abandoned by practically all of the major automobile manufacturers in favour of the methods belonging to the second category.
For the above purpose, the advent of first-generation common-rail injection systems, capable of performing two consecutive injections in each cylinder during one and the same engine cycle, has enabled progress to be made in the field of regeneration of diesel-particulate filters.
For example, in the patent application PCT/IT95/00124 filed in the name of the present applicant on Jul. 21, 1995 and published on Feb. 8, 1996 under No. WO 96/03571 there is proposed a strategy in which the increase in temperature of the exhaust gases at inlet to the diesel-particulate filter is obtained by performing, in addition to the main injection, a post-injection during the expansion phase. The phasing of the post-injection with respect to the main injection and the amount of fuel injected are determined in such a way that the combustion of fuel in the expansion phase is such as to determine an increase in the temperature of the exhaust gases sufficient to cause self-triggering of regeneration of the diesel-particulate filter.
A different strategy for obtaining the increase in temperature of the exhaust gases at inlet to the diesel-particulate filter envisages, in addition to the main injection, a post-injection performed during the exhaust phase. In fact, since in general the diesel-particulate filter is integrated in a single canister together with an oxidizing catalytic converter for abating the unburnt hydrocarbons and carbon monoxide, set upstream of the diesel-particulate filter itself, a fuel post-injection performed prevalently in the exhaust phase of the engine means that the fuel injected will not contribute, except to a reduced extent, to combustion, generating a motive torque, and hence will reach the catalytic converter directly in the unburnt state, thus determining an increase in the hydrocarbons HC present in the exhaust gases, which, in turn, are oxidized within the oxidizing catalytic converter. The exothermic reaction of oxidation that occurs within the oxidizing catalytic converter brings about the rise in temperature of the exhaust gases at inlet to the diesel-particulate filter, which, as is known, is located downstream of the oxidizing catalytic converter, thus enabling regeneration of the diesel-particulate filter itself.
Enormous headway in the field of regeneration of diesel-particulate filters has then been achieved with the advent of second-generation common-rail injection systems, which are able to perform up to six consecutive injections in each cylinder during one and the same engine cycle. For a more detailed treatment of the subject regarding multiple injections, reference may be made to the European patent application No. EP 00104651.5 filed on Mar. 3, 2000 in the name of the present applicant and published on Sep. 13, 2000 under No. EP 1035314, the contents of which are considered entirely incorporated herein.
For example, in the European patent application No. EP 02017387 filed in the name of the present applicant on Aug. 2, 2002 and published on Feb. 5, 2003 under No. EP 1281852, it is proposed to obtain the rise in temperature of the exhaust gases by simply varying the phasing of one or more of the injections with respect to the phasing of the injections themselves in conditions of non-regeneration of the diesel-particulate filter.
First-generation diesel-particulate filters are made using ceramic materials with a much higher thermomechanical resistance (silicon carbide), and these characteristics of strength enable management of regeneration in a not particularly accurate way but with the sole purpose of performing regeneration in the shortest time possible.
Consequently, the strategies of regeneration currently implemented in vehicles are conceived in such a way as to speed up as much as possible the step of warming-up of the diesel-particulate filter providing, in some cases, a sort of overboost of temperature such as to speed up light-off of the particulate.
This type of approach presents as contraindication the onset of high temperature peaks and of high thermal gradients, which, in particularly critical cases, can damage the diesel-particulate filter or even cause its failure. Failure of this sort is normally compound and hence does not cause deterioration in the filtering efficiency according to the Euro-4 standard. However, in future, with the advent of the new standards, probably the vehicles that will undergo failure of the diesel-particulate filter will no longer be able to meet the limits envisaged by law.
In addition, should the aim be to create diesel-particulate filters with less strong ceramic materials, and hence less costly than the ones currently used, the present mode for managing regeneration of the diesel-particulate filter would not guarantee reliability over time thereof.
The aim of the present invention is to provide a method for managing regeneration of a diesel-particulate filter that will enable the drawbacks indicated above to be overcome at least in part.
Provided according to the present invention is a method for managing regeneration of a diesel-particulate filter as defined in the annexed claims.